This invention relates to an apparatus provided with a magnetic recording and reproducing device capable of writing and reading information in and from a magnetic recording medium such as a magnetic tape by a magnetic head in which a write core and a read core are wound around a single core.
In recent years, the standardization of a novel photographing system has been promoted. In a film used for this novel photographing system, strip-like magnetic recording tracks (hereinafter, referred to as "magnetic recording portion") are provided in parallel to the exposure areas of the respective frames at one side along the length of the film. Pieces of information concerning the photographing such as a date of photographing and an exposure value (hereinafter, "film photographing information") are magnetically recorded in the magnetic recording portions in correspondence with the frames.
In the film used for the novel photographing system, a light blocking lid is provided at a film outlet of a film cartridge as disclosed in, e.g., U.S. Pat. No. 5,347,334 and the film is completely contained in the film cartridge. Further, a bar code plate by which the state of the contained film (an unexposed film, an exposed film, a film which is exposed halfway (hereinafter, "a partly exposed film), a developed film, etc.) is detectable is integrally rotatably provided at one end of a spool. The state of the film can be distinguished by reading a bar code on the bar code plate. Accordingly, even if the partly exposed film is rewound into the film cartridge and taken out of the camera, it can be loaded in the camera again to record images in unexposed frames. Further, a developed film can be stored by being contained in the film cartridge.
As described above, with the novel photographic system, a partly exposed film can be loaded in the camera and exposures can be made to unexposed frames. In order to realize such a function, there has been proposed a camera provided with a frame judging function: a signal is detected by scanning each magnetic recording portion of the loaded film by a magnetic head and whether or not the frame corresponding to the magnetic recording portion is exposed is judged based on whether or not the detection signal is a magnetic signal. As a magnetic recording and reading device of this camera, a magnetic head of core share type in which a write coil and a read coil are wound around a single core is used.
FIG. 18 is a circuit construction diagram of writing and reading portions arranged in the vicinity of a magnetic head in a prior art magnetic recording and reproducing device.
In FIG. 18, a magnetic head 100 is of core share type in which a ring shaped core 102 is formed with a gap 101 and a write coil 103 and a read coil 104 are wound around the ring-shaped core 102 at the opposite sides of the gap 101. The write coil 103 is connected with a writing portion 105, whereas the read coil 104 is connected with a reading portion 106. In the position of the gap 101, a magnetic recording medium 109 to the surface thereof magnetic substance is applied such as a magnetic tape and a magnetic film is relatively movably arranged.
The writing portion 105 has input terminals a, b. The input terminal a is connected with transistors Tr3, Tr4 and a resistor R10 via a resistor R12, whereas the input terminal b is connected with transistors Tr5, Tr6 and a resistor R11 via a resistor R13. Between a power source Eg and the collector of the transistor Tr3, a series circuit constructed by resistors R2, R3 and a transistor Tr1 having its base connected with a midpoint of the resistors R4, R5 are connected in parallel. Between the power source Eg and the collector of the transistor Tr5, a series circuit constructed by resistors R4, R5 and a transistor Tr2 having its base connected with a midpoint of the resistors R2, R3 are connected in parallel. A series circuit constructed by a resistor R1 and the write coil 103 is connected between the collectors of the transistors Tr3, Tr5.
In the above construction, when a high signal is input to the input terminal a (or a low signal is input to the input terminal b), a constant current I1 flows into a ground (GND) from the power source Eg via the transistor Tr2, the write coil 103, the resistor R1, the transistor Tr3 and the resistor R10. Conversely, when a high signal is input to the input terminal b (or a low signal is input to the input terminal a), a constant current I2 flows into the ground (GND) from the power source Eg via the transistor Tr1, the resistor R1, the write coil 103, the transistor Tr5 and the resistor R11. By generating magnetic fluxes in opposite directions at the gap 101, each magnetic data can be magnetized (written) in the magnetic recording medium 109 in a direction corresponding to the direction of the magnetic flux.
In the reading portion 106, a series circuit constructed by resistors R8, R6 and a series circuit constructed by resistors R9, R7 are connected between a bias power source Ed and a ground (GND), and the midpoints of the respective series circuits are connected with the read coil 104 and an amplifier AMP for reproduction. An ac signal detected by the read coil 104 is output to the amplifier AMP after being superimposed on a bias voltage, and is extracted after being amplified by the amplifier AMP.
In the case that the magnetic head 100 of core share type is used, the write coil 103 and the read coil 104 are magnetically connected by the ring-shaped core 102. A current flow in one coil generates an induction voltage in the other coil.
While the information is read, the transistors Tr1 to Tr6 of the writing portion 105 are turned off, and an impedance at a writing side becomes high. However, a small amount of leakage current is present in the write coil 103. Even if only a very small amount of current flows in the write coil 103, the induction voltage generated due to the mutual induction action is not completely negligible since the gain of the amplifier AMP of the reading portion 106 is set considerably high in view of the instability of the magnetic signal level. Particularly in the case that the level of the magnetic signal is approximately as low as a noise level, the noise coming from the write coil 103 by the mutual induction action makes it difficult to accurately detect the magnetic signal. Especially in the case that the frame judgment is made based on the magnetic signal, the judgment result is considerably reduced.